Ajax to list products to add to the bundle forbidden 403 until I login in admin

Hi there,
I don’t have screenshot at the moment, but I’ll post it as soon as the problem re-appears.

My client was noticing that from time to time when someone tries to add a product to the bundle, the loading icon was spinning endlessly. I looked into that and I found that the ajax that retrieves the possible products (bopobb_product_list) is failing with status 403. (Example link added in the site info section)

The problem disappears as soon as someone logs in the WordPress admin.

What could it be?
Thanks